a. Field of the Invention
The present invention relates to a method of producing a hydrocarbon liquid polymer, and to be precise, it relates to a method of obtaining a liquid polymer having a high viscosity index, low pour point and very low viscosity by effecting the polymerization of a feed olefin having more than 6 carbon atoms in the presence of a specific catalyst.
B. Description of the Prior Art
The currently available lubricating oils can be broadly classified into the petroleum lubricating oils, synthetic lubricating oils, fatty oils, etc. on the basis of the starting material used for manufacturing thereof, but with the recent increase of the range of use of these lubricating oils, the fields needing lubricating oils with properties meeting the requirements of high viscosity index, low pour point, etc. are increasing. For instance, in the fields where the heat resisting property and low-temperature stability are required of gas turbine oil, refrigerating machine oil, etc. or in fields where the gear oil, motor oil, grease, etc. are required to be free of constant maintenance work, the demand for such lubricating oils has increased. However, the petroleum lubricating oils -- which have the widest range of application at present and are usually obtained from petroleum -- do not have all of such properties as above to perfection as required for said specific use.
On the other hand, among the synthetic lubricating oils produced by organic synthesizing methods, the liquid polymers or olefin polymer oils to be obtained by polymerizing olefins have recently become the object of public attention. And, as the method for the polymerization of olefins, the cationic polymerization method employing Lewis acids like aluminum chloride and the radical polymerization method employing heat or peroxides are popular. However, the polymerizations by these conventional methods are accompanied by isomerization during the reaction, and in fact, there has not yet been obtained a polymerized oil having as high a viscosity index as is required. Under such circumstances, in the production of olefin polymer oils, the application of the so-called Ziegler catalyst which is a coordinated anionic catalyst and is capable of effecting regular polymerization is nowadays in the limelight, and various polymerization reactions employing varieties of olefins as raw material have been tried: for instance, the application of a complex compound prepared from monoethylene aluminum dichloride and titanium tetrachloride as catalyst in polymerizing olefins having 6 to 10 carbon atoms has proved effective for producing liquid polymers having tolerably superior properties, such that the viscosity index is more than 130, the flash point is more than 210.degree.C and the pour point is less than -50.degree.C.
However, said Ziegler catalyst is defective in that (1) the active ingredient of this catalyst is unstable so that it is difficult to recover it for reuse, (2) the catalyst per se is expensive, (3) the handling of the catalyst is attended with danger so that it is necessary to take intensive safety measures, and (4) it is difficult to synthesize a polymerized oil having low grade of viscosity and high efficiency by the use of this kind of catalyst.
Polymerized oils with low grade of viscosity are especially important as they have a wide range of application as gas turbine oil, working oil for aircraft and substitute for squalene, and accordingly, various improvements thereof have been tried. Nevertheless, as there has not been found any apposite catalyst to date, it is usual to prepare a polymerized oil with relatively high viscosity by the use of a Ziegler catalyst, fractionate said polymerized oil by molecular distillation or the like, and employ a portion of the resulting fractions. But, the yield of the fraction having the required properties is no more than 30 to 40%, and as long as the residual fractions equivalent to 60 to 70% are not utilized effectively, this measure cannot be called an effective method. Accordingly, there has been a demand for the development of such a catalyst as will render it possible to produce polymerized oils with low grade of viscosity through mere polymerization.
In this connection, there have admittedly been some attempts to recover the applied catalyst for reuse. For instance, Japanese Patent publication No. 3804/1969 discloses a method comprising dissolving excess aluminum chloride in a complex composed of aluminum chloride and ethyl acetate ester, effecting the polymerization of olefin by the use of the resulting solution, separating the catalyst by settling, and recovering the thus separated catalyst for reuse. U.S. Pat. No. 2697694 discloses a method of obtaining a macromolecular polybutene, which comprises dissolving excess aluminum chloride in a mixture of aluminum chloride and acetone and effecting the polymerization of isobutane by the use of the resulting solution, and in the case of this method too, the applied catalyst is recovered for reuse in the same way as in the foregoing method. However, both of these methods are alike in the application of the cationic polymerization employing aluminum chloride, and neither of them can produce a polymer of olefin having a relatively low viscosity and high viscosity index.